1. Field of the Invention
The present invention relates to a method of measuring the deviation of the run of the actual vertical center axis of fluid filled bore holes relative to the direction the design center axis thereof, the excavation of said bore holes and trenches proceeding by utilization of a bentonite fluid filling substantially completely said bore holes and said trenches such to back the side walls of said bore holes and said trenches to prevent a cave-in thereof, said method comprising an insertion of a measuring cable means into said bore hole or said trench whereby the lower end of said measuring cable means inside said bore hole or said trench is aligned at a first horizontal measuring plane with the center of said bore hole or said trench and thereafter at a predetermined point in a second horizontal measuring plane at or above ground level the horizontal deviation of the position of said measuring cable means relative to the run of the true vertical design centerline of said bore hole or trench is determined. The present invention relates further to an apparatus for carrying out said method.
2. Description of the Prior Art
Vertical bore holes and trenches of variable cross-sectional sizes and variable depths are excavated into soil with the use of bentonite slurry to realize underground structures known as diaphragm walls. Said bore holes or sections of trenches are excavated and then cast with concrete or another suitable building material one after the other to form a continuous underground wall of considerable depth such as up to e.g. 400 feet. During the excavation such bore holes or trenches are kept filled with bentonite slurry, having the function of supporting the soil, i.e. the inner side walls of the bore holes or trenches such to prevent a cave-in thereof.
In many instances said underground walls must be installed with an extremely high accuracy regarding their vertical alignment. To this end specialized trenching equipment is used and particular excavation methods are adopted.
However, as the excavation proceeds periodic measurements of the verticality of the bore hole or trench are required to detect any tendency of a deviation of the centerline of the excavated portion of the bore hole or trench from the true vertical line such that if necessary timely appropriate measures can be taken to rectify wrong alignments.
Obviously, the accuracy in keeping the vertical alignment depends, apart from the excavation procedure itself, also substantially from the accuracy of the measurement of the deviation itself. Therefore, if strict or severe tolerances regarding the verticality are imposed it is of paramount importance that the method and apparatus utilized for measurements as to the verticality are such that a high degree of accuracy of the measurement itself is guaranteed.
The prior art knows various methods for the measuring of the vertical alignment of a bore hole. Most of these known methods are, however, developed and suitable for measurements in bore holes of a rather small diameter relative to their depths. The methods use instruments which can measure the angle of deviation from the true vertical of the actual centerline of the bore hole, at the depth where such instruments are placed, as well as the direction of the horizontal projection of the same bore hole centerline.
In order to locate the exact three-dimensional position of the center at the bore hole bottom a succession of several measurements at short intervals along the length of the bore hole is required. However, with such method the accuracy in the positioning is to a large extent affected due to the high number of measurements involved.
Furthermore, it has been experimented that if a measurement cable is immersed in a viscous fluid such as bentonite slurry and is tensioned vertically, the perfect alignment of said cable with the true vertical is reached asymptotically in a rather long time due to the reaction or resistance of the fluid which opposes the horizontal movement of such cable.
The time span required by a given cable to stabilize itself in the final position is a function of its length, its diameter and the tensioning force applied at this cable. In order to reduce the time span for conducting the measurement and to increase the accuracy of the position of the cable it is desirable to reduce the diameter of the cable itself or to increase the tensioning force acting on such cable.
The methods of the prior art have not considered the tensioning force of the cable as relevant because of the general low accuracy of the known procedures. In the known methods the the measurement cable is simply kept in tension by a weight device suspended to it, such as a plumb. For practical reasons the weight of such device cannot exceed about one-half of the breaking strength of the cable, such that the tensioning thereof is quite limited and the measurement carried out thereby inaccurate and time consuming.
In the known methods, such as described above, where the tensioning force acting on the cable is rather low, to measure the verticality of the cable this must be at sight for a rather long length, which means that it has to be suspended high above ground level. The measurement is then made by means of a theodolite instrument, placed in two positions in orthogonal alignment with the cable.